Streptococcus is a class of spherical Gram-positive bacteria that cause many diseases in humans and animals. Streptococcus pneumoniae is the main pathogenic bacteria causing lobar pneumonia and other diseases. It more frequently causes bronchopneumonia among infants and children. Streptococcus pyogenes is widely distributed in nature, and is one of the most important human bacterial infection pathogens. It can cause a variety of suppurative inflammation, including scarlet fever, erysipelas, neonatal sepsis, meningitis, Streptococcus allergy and so on. Group B Streptococci, such as Streptococcus agalactiae, is one of the important pathogens of perinatal infection, which seriously threatens the health of the newborns. Streptococcus. mutans is the main pathogen of dental caries, which seriously threatens the oral health of human beings. At present, there is no effective vaccine for the prevention and treatment of dental caries. In addition, Streptococci can cause a number of important animal diseases and threaten animal husbandry seriously. Streptococcus suis disease is a zoonotic acute and febrile infectious disease, usually caused by group C, D, E, and L Streptococci. Mastitis, the most important disease of farming industry in dairy cows, is caused by four main pathogenic bacteria, Streptococcus agalactiae, Streptococcus uberis, Streptococcus dysgalactiae and Staphylococcus aureus. However, there is still lack of effective vaccine and treatment strategy today. From the above, we can see that Streptococcus species are varied and can cause a large number of diseases of human beings and animals.
Phage lysin is a type of cell wall hydrolase expressed in late stage after dsDNA phage infecting host bacteria. Usually the size of the lysin is 25 kD˜40 kD. Structurally, lysin is composed of two independent functional domains, the N-terminal catalytic domain and the C-terminal cell binding domain (CBD) that determines cell binding sites. The two domains are linked by a small fragment. Sequence analysis shows that the catalytic domain of the same type of lysin is highly conserved, while the cell binding domain is variable, which provides the possibility to construct a new chimeric lysin. Lysins are highly specific and can only specifically identify and lyse specific species of bacteria. In addition, the cleavage site is very well conserved, and the specificity of the phage and bacterial coevolution makes it difficult for host bacteria to develop resistance to them. These lysins characteristics provide theoretical feasibility for the control and treatment of drug-resistant bacteria in clinical practice. Until now, a few natural lysins that acts upon Streptococci have been reported. These lysins can kill specific strains of Streptococci both in vivo and in vitro. However, these lysins usually have a narrow lytic spectrum, and most of them are difficult to be solubly expressed, or do not high active, and active in a narrow range of pH, generally in pH 5-8. At present, none of lysins reported is able to lyse Streptococcus mutans effectively. It is very importantly significant to look for lysins, which are soluble, able to be expressed in high level, and show broad spectrum of activity, for development of new anti-streptococcal drugs and to control of streptococcal infections in vitro and so on.